Substation Grounding / Earthing

Substation Gronding/ Earthing

The sole purpose of substation grounding/earthing is to protect the equipment from surges and lightning strikes and to protect the operating persons in the substation. The substation earthing system is necessary for connecting neutral points of transformers and generators to ground and also for connecting the non current carrying metal parts such as structures, overhead shielding wires, tanks, frames, etc to earth. Earthing of surge arresters is through the earthing system. The function of substation earthing system is to provide a grounding mat below the earth surface in and around the substation which will have uniformly zero potential with respect to ground and lower earth resistance to ensure that

To ensure safety of the operating staff by limiting voltage gradient at ground level in the substation

To provide low resistance path to the earthing switch earthed terminals, so as to discharge the trapped charge (Due to charging currents even the line is dead still charge remains which causes dangerous shocks) to earth prior to maintenance and repairs.

Earth Resistance

Earth Resistance is the resistance offered by the earth
electrode to the flow of current in to the ground. To provide a sufficiently
low resistance path to the earth to minimize the rise in earth potential with
respect to a remote earth fault. Persons touching any of the non current
carrying grounded parts shall not receive a dangerous shock during an earth
fault. Each structure, transformer tank, body of equipment, etc, should be
connected to earthing mat by their own earth connection.

Generally lower earth resistance is preferable but for
certain applications following earth resistance are satisfactory

Large Power Station s– 0.5 Ohm

Major Power Stations - 1.0 Ohm

Small Substation – 2.0 Ohm

In all Other Cases – 8.0 Ohm

Step Potential and Touch Potential

Grounding system in an electrical system is designed to achieve low earth resistance and also to achieve safe ‘Step Potential ‘and ‘Touch Potential’.

Step Potential:

Step potential is the potential difference between the feet of a person standing on the floor of the substation, with 0.5 m spacing between the feet (one step), through the flow of earth fault current through the grounding system.

Touch Potential:

Touch potential is a potential difference between the fingers of raised hand touching the faulted structure and the feet of the person standing on the substation floor. The person should not get a shock even if the grounded structure is carrying fault current, i.e, The Touch Potential should be very small.

Step Potential and Touch Potential

Types of Grounding:

Un earthed Systems:

It is used no more. The neutral is not connected to the earth, also called as insulated neutral system.

Solid grounding or effective grounding:

The neutral is directly connected to the earth without any impedance between neutral and ground.

Resistance grounding:

Resistance is connected between the neutral and the ground.

Reactance grounding:

Reactance is connected between the neutral and ground.

Resonant Grounding:

An adjustable reactor of correctly selected value to compensate the capacitive earth current is connected between the neutral and the earth. The coil is called Arc Suppression Coil or Earth Fault Neutralizer.

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